Pressure measurements of fluid flowing within a tube can be made in a variety of ways. For example, a strain gage, may be placed on the outside of the tube. However, as internal pressure of a tube changes, the tube "balloons," i.e., the tube wall stretches, varying the tube wall thickness, in the area where the strain gage is located. Forces due to the internal pressure of the tube, and forces due to variations in thickness along the tube wall, hinder accurate measurement of the internal pressure alone.
Another method uses a "T" fitting to divert a portion of the fluid to a pressure transducer. This method adds cost, complexity, and increases the probability of leakage, and may produce trauma to the fluid (if the fluid is blood, trauma can result in hemolysis).